1. Field of the Invention
This invention relates to a method and apparatus for generating and detecting servo marks and, more particularly, to a technique for detecting a servo mark pattern as a unique pattern of data sequences and for generating a timing reference therefrom.
2. Description of the Prior Art
In the prior art, a magnetic disk having so-called sectored or embedded servo information contains servo information which is utilized for positioning or re-aligning a recording-reproducing head or heads, on the same disk surface as data information. More specifically, the magnetic disk typically contains a plurality of tracks each of which is divided into a plurality of sectors. Each sector in the embedded servo system contains servo, informational, address and synchronization data and may, for example, be formatted as shown in FIG. 1. That is, each sector includes a recovery area, a servo information section and a read/write section.
The recovery portion provides the time needed for the circuitry contained within the associated disk drive unit to change modes as, for example, between a write mode in a preceding sector and a read mode for reading servo information in the current sector. As is to be appreciated, while the circuitry is changing modes, data are not properly read and processed. Therefore, since the disk is continuously rotating, the sector portion which occurs during the time required for the mode change is unavailable for data transfer. As a result, the recovery portion does not contain any servo or read/write data and, as an example, may just contain a pre-recorded high frequency signal.
The servo information section of the sector includes a DC erase portion, a track address portion and a servo positioning burst portion. The DC erase portion typically includes a series of blanks or "0" bits which exceed the number of consecutive "0" bits permitted to occur in other areas of the sector. As a result, when this number of "0" bits is detected by a servo control circuit contained within the disk drive unit, the servo system is conditioned for locating the servo burst data as hereinafter described. The track address portion contains data representing the current track number which, for example, may be expressed in Gray code. The servo positioning burst portion 19 contains servo burst data as, for example, shown in FIG. 2, which are utilized for detecting tracking errors and for re-aligning the recording/reproducing head or heads. That is, any positional misalignment relative to the track being scanned is determined upon reading the servo burst data. The misalignment information is supplied in a closed loop feedback path to the servo control circuit, whereupon the position of the head or heads is appropriately re-adjusted or corrected.
The read/write section of a sector includes an address mark, a synchronization portion, an identifying number and an informational data portion. The synchronization portion contains synchronization data which when provided to a phase locked loop circuit contained in the disk drive unit enables an internal clock or voltage controlled oscillator to be synchronized to the rate at which data are transferred to and from the disk. The identifying portion includes data which represents a sector identifying number. Thus, by utilizing data from the track address and the identifying portions, a desired information data portion can be located. The informational data portion is utilized for writing and reading therefrom user generated data.
It is seen that, in the prior art embedded servo system, upon completing the writing of data into an informational data portion of a preceding sector, a change-over delay in the form of the recovery area allows the disk drive circuitry to change from a write mode to a read mode. Afterwards, if the DC erase portion is properly detected, a timing reference is established from which the servo burst data may be located in either a synchronous or an asynchronous manner. A typical synchronous approach utilizes a phase locked loop data field recorded in the servo information section for synchronizing a voltage controlled oscillator to the rate at which data are read from the phase locked loop data field. As is to be appreciated, the area required for the phase locked loop data field reduces the area available in the sector for informational data. Further, the use of a phase locked loop circuit increases the overall circuit complexity and cost. Moreover, since during seek operations (i.e. the locating of a desired track or sector) the read data may have a relatively large variation in amplitude and phasing, obtaining a proper phase locking may be difficult.
On the other hand, in a conventional asynchronous approach, upon the detection of the DC erase portion, a trigger signal is generated from which a predetermined number of clock pulses is counted so as to locate the servo burst data. However, due to defects which may be present in the DC erase portion, the detection of the DC erase portion and the establishing of the timing reference may not always occur at the same relative location in each sector. Therefore, since the number of clock pulses which are counted after the timing reference is fixed, the reading of the servo burst data may not always start at the beginning of the burst portion. As a result, the length of the servo burst portion may be made longer than necessary so as to insure that sufficient servo burst data are always available.
As is to be appreciated, proper detection of the DC erase portion is required to insure the proper subsequent transfer of data. However, the occurrence of defects known as drop-ins (i.e. erroneous extra data pulses which occur during either the recording or reproducing processing of the DC erase portion) may interrupt the uniquely long sequence of "0" bits so that the DC erase portion may appear to contain several shorter sequences of "0" bits and, as such, may prevent the detection of the DC erase portion. If this occurs, a timing reference is not established and, as a result, the servo control circuit does not properly locate the servo burst data and the head or heads may not be properly re-aligned as previously described. In this situation, the servo burst data may be interpreted incorrectly and may, for example, be mistaken for informational data. Further, defects known as drop-outs (i.e. data pulses which drop-out or are "lost" due to defects occurring during either the recording or reproducing processing) which occur in the sector, for example, in the informational data portion, could cause an area within the informational data portion to be mis-interpreted as a DC erase portion. As a result, the servo control circuit attempts to initiate the above-described procedure for locating servo burst data and, instead of servo burst data, receives the remaining informational data, whereupon the servo control circuit attempts to re-align the head or heads based on this incorrect data. Moreover, in this situation, the remaining informational data is not transferred and processed as informational data.
Thus, the prior art has failed to provide a technique for generating a timing reference from which the beginning of the servo burst portion can be readily located, even if defects are present within the sector, while using a minimal amount of sector area and being relatively inexpensive to implement.